The present invention relates to a control system for an automatic transmission and more particularly to a control system which is provided within a hydraulic control system for controlling an automatic transmission for a vehicle using a belt drive continuously-variable speed transmission (CVT) to increase and decrease the reduction ratio of the belt drive continuously-variable speed transmission in response to the running conditions of the vehicle.
A belt drive continuously-variable speed transmission is employed as an automatic transmission for a vehicle in combination with a torque converter or a fluid coupling and a changeover mechanism including a forward drive and a reverse drive. such an automatic transmission is controlled by a hydraulic control system which employs the running conditions of the vehicle, such as the running speed of the vehicle, the number of revolutions of an input pulley, the degree of throttle opening and the like, as input signals and control the supply of a working fluid to as well as the discharge of a working fluid from the belt drive continuously-variable speed transmission, the hydraulic servo-motor of the changeover mechanism including a forward drive and a reverse drive and the fluid coupling, and the supply of lubricating oil to various parts of the automatic transmission. This hydraulic control system is provided to control, in response to the running conditions of the vehicle, the belt drive continuously-variable speed transmission comprising an input pulley and an output pulley variable in effective diameter by the hydraulic servomotor provided on an input shaft and an output shaft, respectively, and a V-belt extended between both the pulleys. As for one example, the hydraulic control system comprises a regulator valve for regulating the discharge oil pressure of an oil pump in response to the input oil pressure in connection with the throttle pressure, the running speed of the vehicle or the reduction ratio to release it as a line pressure, a throttle valve for regulating the supplied line pressure in response to the degree of throttle opening to release it as a throttle pressure, a reduction ratio detection valve for regulating the supplied line pressure in response to the running speed of the vehicle or the reduction ratio of the belt drive continuously-variable speed transmission to release the oil pressure in connection with the running speed of the vehicle or the reduction ratio, and a reduction ratio control valve provided with a spool actuated by two control oil pressures applied opposedly each other to supply said line pressure of the hydraulic servomotor of said input pulley and the discharge pressure of said hydraulic servomotor. An electronically-controlled automatic transmission for a vehicle further comprises two solenoid valves which are controlled by an electric control circuit which outputs in response to the running conditions of the vehicle such as the running speed of the vehicle, the number of revolutions of the input pulley, the degree of throttle opening and the like and regulate two control oil pressures of said reduction ratio control valve. However, the conventional reduction ratio control valve has been suffered from disadvantages in that since sufficient consideration to cope with the rapid increase in degree of throttle opening, when the so-called kickdown takes place whereby an accel pedal is rapidly trod to increase the reduction ratio to rapidly increase the output torque in order that the vehicle need be accelerated rapidly or need be run on a steep slope, the oip pressure of the hydraulic servomotor of the input pulley drops to approximately zero, tension of the V-belt temporarily weakens, the V-belt slips due to the reduction in frictional force between the V-belt and the pulley, and the pulley grips the V-belt when the shift is shifted to the upshift after termination of the kickdown shift, and therefore, a violent shock likely occurs.